DENT 1200 Dental Materials I

Lesson 3 Gypsum

1. Identify the Source and Color of pure raw gypsum.

• The raw form of gypsum is obtained from gypsum rock called alabaster. The pure rock is white and translucent. It may be tinged with gray or yellow due to impurities.

• The composition is crystalline in structure (a solid with regular arrangement of atoms). There are three main forms of gypsum products used in dentistry: Type II plaster, Type III dental (lab) stone and Type IV improved (die) stone. Chemically, plaster and dental stone are identical. Physically the shape and density of the powder particles differ.

1.

2. Chemical formula for raw and calcined gypsum

• RAW• Calcium Sulfate

Dihydrate• CaSO4 2H2O

• Calcined: a process to remove water from raw gypsum and make into powder form

• Calcium Sulfate Hemihydrate

• CaSO4 ½ H2O

3.W/P ratio for 3 types

• Type II Plaster 50/100

• Type III Stone 33/100

• Type IV Improved or Die Stone 22/100

• Different amount of water is needed because particle size and shapes are different due to the calcination process

4. Uses of three types

• Type II study or diagnostic model, to articulate

• Type III cast for fabrication of removables

• Type IV cast of removable dies for fixed

and final impression denture pour

Articulated Model

• May need a bite registration to achieve this.

• Maxillary and Mandibular arches attached to simulate patient bite.

• 1 or more teeth can be made removable dies.

5.Precaution for Use and Storage

• Wear mask and eyewear as this material is an irritant to the lungs.

• Can be very drying to the skin (hands)

• Keep storage bins closed- or water content changes and material does not react as it should

6. Mixing techniques

• 1. Rubber bowl and lab spatula

• 2. Mechanical spatulation

• 3. Vacuum investor

All techniques require a vibrator to remove air voids and increase flow of the material.

Mechanical Vacuum

7. Initial Set and Final Set

• Initial set• Time between the

spatulation and the “loss of gloss”

• Several minutes

• Final Set• Material is hard( can

not indent a fingernail) , dry and cool (heat has been given off in the EXOTHERMIC reaction)

• 45 minutes – 1 hour

8. Methods to Accelerate and Inhibit the set

ACCELERATE_ speed

• Potassium sulfate• Sodium chloride(salt)• Slurry- from unclear

bowl or spatula• HEAT

INHIBIT slow

• Borax• Hydrocolloid gels• Sodium citrate

• BLOOD• COLD

9. Strong and Accurate

• Follow manufacturers directions, including measuring water and powder, place water in bowl first, do not add extra water or powder once the mix has begun.

A thin mix is weaker.

Add the material in increments and vibrate!

IMPRESSION MODEL/CAST

• IMPRESSION- negative reproduction

• CAST/MODEL

positive reproduction

fabricate/study